Hint: Johny's Locations

In order to acquire all of the Chaos Emeralds in the game, you must race a character named Johny multiple times, winning the races. Here are the seven locations you can find him in:

Northeast of Haunted Ship

Northeast of Southern Island

Northwest of Machine Labyrinth

Northwest of Plant Kingdom

Southeast of Machine Labyrinth

Southeast of Southern Island

Southwest of Sky Babylon

Unlockable: Blaze

To unlock Blaze as a playable character, beat the boss of World 3 (Coral Cave).

Unlockable: Music & Dialogue

You can find a koala playing a guitar on Windmill Island after beating the boss in Big Swell. By speaking with him, you can listen to all of the in-game music and dialogue, as well as more missions that will unlock music that would otherwise be unattainable.

Unlockable: New Ending

To unlock a new ending in the game, you must acquire all Chaos and Sol Emeralds in the game.